This invention relates to a fluid-operated drive unit for welding apparatus.
The drive unit employs fluid, specifically air, under pressure to slow the advance stroke of an electrode toward a workpiece to achieve a xe2x80x9csoft-touchxe2x80x9d by the electrode.
The drive unit includes aligned cylindrical wall means preferably forming three chambers or sections with a piston rod extending through the first two chambers. The piston rod has a first drive piston and a floating piston in the second chamber and a second drive piston in the third chamber. The piston rod extends out of the first, cushioning chamber to an electrode holder. The advance stroke of the electrode is achieved through the drive pistons and the stroke is slowed before the electrode contacts the workpiece to provide a soft-touch engagement of the electrode and the workpiece. To achieve this, cushioning air under pressure is supplied from the first chamber through ports in a passage in the piston rod to a forward portion of the second chamber. This air slows the advance of the floating and front drive pistons. The floating piston then moves over and closes a port of the piston rod passage to shut off cushioning air to the second chamber.
Air is vented from this chamber to lower the pressure to atmospheric and thereby enable full welding pressure of the electrode on the workpiece to be applied by the first and second drive pistons.
To return the piston rod to the retracted position, air is supplied under pressure to the forward portion of the second section which moves the floating piston back to the first drive piston and enables air under pressure again to be supplied to the second section from the cushioning section. This air further aids in the retraction of the piston rod to complete the cycle.
The drive unit uses pressurized air to provide the cushioning effect regardless of the rate of flow of the air either into or out of the cushioning chamber. The design also enables the pressurized air in the cushioning chamber to assist in the retraction of the piston rod and pistons to the retracted position. The design also enables a soft touch to be achieved with a faster cycle time because the design enables a high flow rate of the pressurized air.
It is, therefore, a principal object of the invention to provide a soft-touch pneumatic drive unit for welding apparatus which has the objects and advantages discussed above.